Conventionally, in a phase-change optical recording medium where information is recorded and reproduced onto/from a recording layer and the information is rewritable, the recording layer contains four elements, Ag, In, Sb and Te, as primary components. The objective is to improve the recording linear velocity on the basis of stable signal processing, making it possible to stably perform recording and reproducing at 1×-speed to 4×-speed of a recording linear velocity of DVD-ROM (3.49 m/s), which is now commercially available.
In the optical recording medium, the manner in which to conduct heat subtly changes depending upon an optical constant or film thickness of each layer laminated on a substrate, and may greatly affect the recording characteristic in a mark recorded on the recording layer, or reflectance or modulation factor may change. As in Patent Literature 1, the film thickness of each layer laminated onto a substrate, an optical constant of a crystalline phase and an amorphous phase in the recording layer, an optical constant regarding a protective layer and a reflective layer, and the condition for groove depth of a transparent substrate are important factors relating to recording characteristics and signal processing.
In addition, in order to realize high-speed recording, recording must be performed with additionally higher power, so the storage reliability becomes more severe than in comparison with the conventional art. Using Ag having high thermal conductivity for a reflective layer binds with S in ZnS—SiO2 of a protective layer and Ag2S is formed, so it is necessary to establish a sulfidation prevention layer between the reflective layer and the protective layer. However, recording with higher power causes the easy occurrence of peeling-off with interference between the Ag reflective layer and the sulfidation prevention layer, leading to the problem that a disc defect easily occurs after repeated recording or after long-term storage in a severe environment.
As a related well-known technology, Patent Literature 2 discloses an invention where tantalum oxide, tantalum and nickel are used for an intermediate layer making contact with an Ag reflective layer, and Mw(SbzTe1-z)1-w, provided that 0≦w≦0.3 and 0.5≦z≦0.9, for a recording layer. However, the invention has different constituent elements from those in the sulfidation prevention layer of the present invention; and in addition, recording is performed at 2×-speed of CD (2.4 m/s), which is at a lower velocity, and lower density compared to those in the present invention.
Further, Patent Literature 3 relating to the present application by the applicant discloses an invention where Si, SiC, Ge and GeCr are used for a sulfidation prevention layer making contact with an Ag reflective layer, and GaαGeβInySbδTeε (provided that 0<α<7, 0<β<10, 0<γ<5, 60<δ<80 and 0<ε<5) is used for a recording layer. However, the invention has different constituent elements from those in the sulfidation prevention layer of the present invention. In addition, recording is performed at 20 m/s at maximum, which is at a lower velocity compared to that of the present invention.
Further, Patent Literature 4 discloses an invention where a recording layer composed of GeSbIn, and recording is performed at 2.4 m/s to 9.6 m/s, which is at a lower velocity compared to that in the present invention.
The present invention is directed at providing a DVD+RW recordable at 6×-speed to 8×-speed of DVD-ROM (hereafter, simply referred to as ‘6×-speed to 8×-speed), and optical constants in a crystalline state and an amorphous state in a recording layer, and the groove condition to control high reflectance are studied; concurrently, the sulfidation prevention layer is also studied.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-76702    Patent Literature 2: Japanese Patent (JP-B) No. 3494044    Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 2003-248967    Patent Literature 4: Japanese Patent Application Laid-Open (JP-A) No. 2001-39031